The effect of shrubs on tree recruitment differs between planted and secondary forests

Recruitment of tree species is a bottleneck for forest regeneration, particularly in water-limited environments, such as Mediterranean forests. Despite nurse shrubs facilitating tree recruitment in adverse environments, we lack information on the facilitative role of functionally different shrubs while considering previous land uses. This study evaluates the effect of shrubs on tree recruitment in planted and secondary forests along environmental gradients, assessing how recruitment depends on the identity of shrub functional types and the beneficiary tree species. We used the Spanish Forest Inventory database to calculate tree recruitment for main tree species (pines or oaks) and shrub abundance in forests of central Spain. We fitted generalised linear mixed models of tree juvenile abundance and seedling occurrence to test the effect of shrub crown volume and forest type, considering all shrub species and the main families. Forest type (planted or secondary) modulated the relationship between shrub volume and tree juvenile abundance per species, but had a more limited effect on seedling occurrence. In some cases, planted forests reduced the effects of shrub volume on recruitment compared to secondary forests or promoted hump-shaped responses, with a peak at moderate shrub volumes. In general, juvenile abundance was less affected by shrubs than by forest type, being higher in secondary forests, especially for oak species. We also found species-specific relationships of shrub volume with tree recruitment. Occurrence of Q. ilex seedlings was strongly associated with various shrub families, while only one shrub family at most was related to the recruitment of the rest of tree species.